use Logs, change API of Store.Iter and of private stores, Windows support

• Use Logs (#342, @talex5)
• Improve non-unix portablity of Irmin_fs (#345, @samoht)
• Change the signature of Store.iter to defer opening the
  file only when needed. This was causing a file-descriptor
  early exhaustion on Windows (#345, @samoht)
• Fix paths for references on Windows (#345, @samoht)
• Port to ocaml-git 1.8.0
• Rather large API change in Irmin.Private.Contents.Store
  and Irmin.Private.Commit.Store to make it easier to
  build new and efficient Irmin backends. (#346, @samoht)
• Fix performance problem in the computation of LCAs (#351, @talex5)
• Fix sort order for Git trees (#352, @talex5)